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新生儿硬肿症的主要临床表现
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_________ is a service provided by ground-based controllers who direct aircraft on the ground and in the air
En-route air traffic controllers work in facilities called ______.
En-route air traffic controllers issue clearances and instructions for airborne aircraft, and pilots are required to ______ these instructions.
Each ACC is responsible for many thousands of square miles of airspace known as ________ and for airports within that space.
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